Apparatus and method for applying frequency calibration to local oscillator signal derived from reference clock output of active oscillator that has no electromechanical resonator

ABSTRACT

A wireless system includes a local oscillator (LO) signal generation circuit, a receiver (RX) circuit, and a calibration circuit. The LO signal generation circuit generates an LO signal according to a reference clock. The LO signal generation circuit includes an active oscillator. The active oscillator generates the reference clock, wherein the active oscillator includes at least one active component, and does not include an electromechanical resonator. The RX circuit generates a down-converted RX signal by performing down-conversion upon an RX input signal according to the LO signal. The calibration circuit generates a frequency calibration control output according to a signal characteristic of the down-converted RX signal, and outputs the frequency calibration control output to the LO signal generation circuit. The LO signal generation circuit adjusts an LO frequency of the LO signal in response to the frequency calibration control output.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application No.62/620,002 filed Jan. 22, 2018 and U.S. provisional application No.62/642,653 filed Mar. 14, 2018. The entire contents of the relatedapplications, including U.S. provisional application No. 62/620,002 andU.S. provisional application No. 62/642,653, are incorporated herein byreference.

BACKGROUND

The present invention relates to wireless communications, and moreparticularly, to an apparatus and method for applying frequencycalibration to a local oscillator signal derived from a reference clockoutput of an active oscillator that has no electromechanical resonator(e.g., crystal).

A transmitter (TX) circuit in a wireless system chip is used to performan up-conversion process that converts a TX signal from a lowerfrequency to a higher frequency for signal transmission. A receiver (RX)circuit in the wireless system chip is used to perform a down-conversionprocess that converts an RX signal from a higher frequency to a lowerfrequency for signal reception. Further, each of the up-conversionprocess and the down-conversion process requires a local oscillator (LO)signal with a proper LO frequency setting. Typically, the LO signal isderived from a reference clock that is supplied from an off-chiposcillator. For example, the off-chip oscillator is a passive oscillator(e.g., a typical crystal oscillator (XO)). When the wireless system chipis used by an application device, the off-chip oscillator is also usedby the application device due to the fact that the reference clockneeded by the wireless system chip is supplied from the off-chiposcillator (e.g., XO). If the off-chip oscillator can be omitted, theBOM (bill of material) cost and the PCB (printed circuit board) area ofthe application device can be reduced. Thus, there is a need for aninnovative crystal-less wireless system design. Further, the absolutefrequency accuracy needs to be controlled to avoid violating theEuropean Telecommunications Standards Institute (ETSI) or FederalCommunications Commission (FCC) spectrum emission regulation. Thus,there is also a need for an innovative frequency calibration scheme.

SUMMARY

One of the objectives of the claimed invention is to provide anapparatus and method for applying frequency calibration to a localoscillator signal derived from a reference clock output of an activeoscillator that has no electromechanical resonator (e.g., crystal).

According to a first aspect of the present invention, an exemplarywireless system is disclosed. The exemplary wireless system includes alocal oscillator (LO) signal generation circuit, a receiver (RX)circuit, and a calibration circuit. The LO signal generation circuit isarranged to generate an LO signal according to a reference clock. The LOsignal generation circuit includes an active oscillator. The activeoscillator is arranged to generate the reference clock, wherein theactive oscillator comprises at least one active component, and does notinclude an electromechanical resonator. The RX circuit is arranged togenerate a down-converted RX signal by performing down-conversion uponan RX input signal according to the LO signal. The calibration circuitis arranged to generate a frequency calibration control output accordingto a signal characteristic of the down-converted RX signal, and outputthe frequency calibration control output to the LO signal generationcircuit, wherein the LO signal generation circuit adjusts an LOfrequency of the LO signal in response to the frequency calibrationcontrol output.

According to a second aspect of the present invention, an exemplarycalibration system is disclosed. The exemplary calibration systemincludes a calibration signal source and a first wireless system. Thecalibration signal source is arranged to transmit a calibrationreference signal via an antenna. The first wireless system includes afirst local oscillator (LO) signal generation circuit, a first receiver(RX) circuit, and a first calibration circuit. The first LO signalgeneration circuit is arranged to generate a first LO signal accordingto a first reference clock. The first LO signal generation circuitcomprises a first active oscillator. The first active oscillator isarranged to generate the first reference clock, wherein the first activeoscillator comprises at least one active component, and does not includean electromechanical resonator. The first RX circuit is arranged togenerate a first down-converted RX signal by performing down-conversionupon a first RX input signal according to the first LO signal, whereinthe first RX input signal is obtained from the calibration referencesignal received via an antenna. The first calibration circuit isarranged to generate a first frequency calibration control outputaccording to a signal characteristic of the first down-converted RXsignal, and output the first frequency calibration control output to thefirst LO signal generation circuit, wherein the first LO signalgeneration circuit adjusts an LO frequency of the first LO signal inresponse to the first frequency calibration control output.

According to a third aspect of the present invention, an exemplary localoscillator (LO) signal calibration method is disclosed. The exemplary LOsignal calibration method includes: generating an LO signal according toa reference clock, wherein the reference clock is generated by an activeoscillator, and the active oscillator comprises at least one activecomponent, and does not include an electromechanical resonator;generating a down-converted RX signal by performing down-conversion uponan RX input signal according to the LO signal; and generating afrequency calibration control output according to a signalcharacteristic of the down-converted RX signal, wherein an LO frequencyof the LO signal is adjusted in response to the frequency calibrationcontrol output.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a first calibration system according toan embodiment of the present invention.

FIG. 2 is a diagram illustrating a first wireless system according to anembodiment of the present invention.

FIG. 3 is a diagram illustrating the relationship between an LOfrequency of an LO signal and a control code of an active oscillatoraccording to an embodiment of the present invention.

FIG. 4 is a diagram illustrating an output spectrum of a down-convertedRX signal according to an embodiment of the present invention.

FIG. 5 is a diagram illustrating a second calibration system accordingto an embodiment of the present invention.

FIG. 6 is a diagram illustrating a second wireless system according toan embodiment of the present invention.

FIG. 7 is a diagram illustrating a calibration setup environmentaccording to an embodiment of the present invention.

DETAILED DESCRIPTION

Certain terms are used throughout the following description and claims,which refer to particular components. As one skilled in the art willappreciate, electronic equipment manufacturers may refer to a componentby different names. This document does not intend to distinguish betweencomponents that differ in name but not in function. In the followingdescription and in the claims, the terms “include” and “comprise” areused in an open-ended fashion, and thus should be interpreted to mean“include, but not limited to . . . ”. Also, the term “couple” isintended to mean either an indirect or direct electrical connection.Accordingly, if one device is coupled to another device, that connectionmay be through a direct electrical connection, or through an indirectelectrical connection via other devices and connections.

The present invention proposes a crystal-less (XOless) technique for awireless system. For example, the XOless technique may be integrated ina wireless system chip. Since an off-chip oscillator such as a crystaloscillator (XO) is not needed by the proposed wireless system chip, aBOM cost and a PCB area of an application device using the proposedwireless system chip can be reduced. Compared to a reference clockgenerated from an off-chip crystal oscillator, a reference clockgenerated from an on-chip active oscillator may have less stability andaccuracy. The present invention further proposes a low-cost frequencycalibration scheme. Further details of the proposed XOless technique andthe proposed low-cost frequency calibration scheme are described withreference to the accompanying drawings.

FIG. 1 is a diagram illustrating a first calibration system according toan embodiment of the present invention. The calibration system 100includes a calibration signal source 102 and at least one wirelesssystem 104. For clarity and simplicity, only one wireless system 104 isshown in FIG. 1, where the wireless system 104 is a device under test(DUT). The calibration signal source 102 is arranged to transmit acalibration reference signal S_CAL via an antenna 103. For example, thecalibration reference signal S_CAL is a continuous waveform (CW) signalwith a calibration tone (e.g., a CW frequency f₀ with approximate zeroppm frequency error). Unexpected interfaces shall be avoided during thecalibration process. The wireless system 104 receives the calibrationreference signal S_CAL from an antenna 105, and down-converts a receiver(RX) signal obtained from the calibration reference signal S_CALreceived via the antenna 105 to generate a down-converted RX signal. Thewireless system 104 checks existence of the calibration tone in thedown-converted RX signal. Each of the up-conversion process and thedown-conversion process requires a local oscillator (LO) signal with aproper LO frequency setting. If an LO frequency of an LO signal used bythe wireless system 104 is close to the CW frequency f₀ of thecalibration reference signal S_CAL, the calibration tone is within alimited receiver intermediate frequency (IF) bandwidth, and can be foundin the down-converted RX signal. However, if an LO frequency of an LOsignal used by the wireless system 104 is largely deviated from the CWfrequency f₀ of the calibration reference signal S_CAL, the calibrationtone is beyond the limited receiver IF bandwidth, and cannot be found inthe down-converted RX signal. In this embodiment, the wireless system104 performs frequency calibration according to a checking result. Forexample, an internal oscillator of the wireless system 104 is adjustedfor achieving frequency calibration. For another example, an internalfrequency synthesizer of the wireless system 104 is adjusted forachieving frequency calibration. For yet another example, an internaloscillator and an internal frequency synthesizer of the wireless system104 are both adjusted for achieving frequency calibration.

FIG. 2 is a diagram illustrating a first wireless system according to anembodiment of the present invention. The wireless system 104 shown inFIG. 1 may be implemented using the wireless system 200 shown in FIG. 2.For example, the wireless system 200 may be a Radio Detection andRanging (radar) system, such as a frequency modulated continuouswaveform (FMCW) radar system, a phase modulated continuous wave (PMCW)radar system, a micro-Doppler radar system, or a pulse-Doppler radarsystem. For another example, the wireless system 200 may be anautomotive system such as an automotive radar system. In thisembodiment, the wireless system 200 is implemented on a chip 201, andtherefore has a plurality of on-chip components. As shown in FIG. 1, thewireless system 200 includes an LO signal generation circuit 202, atransmitter (TX) circuit 204, a receiver (RX) circuit 206, a switchcircuit (denoted by “SW”) 208, a control circuit 210, and a processingcircuit 212.

The LO signal generation circuit 202 is arranged to generate an LOsignal S_LO according to a reference clock CK_REF. In this embodiment,the LO signal generation circuit 202 includes an active oscillator 214and a frequency synthesizer 216. The active oscillator 214 includes atleast one active component (e.g., transistor(s) and/or amplifier(s)),and does not include an electromechanical resonator such as a crystal, abulk acoustic wave (BAW) resonator, or a microelectromechanical system(MEMS) resonator. That is, the active oscillator 214 is anelectromechanical-resonator-less oscillator (e.g., a crystal-lessoscillator), and does not consist of passive components (e.g.,inductor(s), resistor(s), and/or capacitor(s)) only. For example, theactive oscillator 214 may be an LC oscillator having an amplifiercircuit and an LC frequency-selective network, where the LCfrequency-selective network consists of on-chip passive components only,and is used to create a resonator needed for reference clock generation.For another example, the active oscillator 102 may be an RC oscillatorhaving an amplifier circuit and an RC frequency-selective network, wherethe RC frequency-selective network consists of on-chip passivecomponents only, and is used to create a resonator needed for referenceclock generation. To put it simply, the active oscillator 102 is anon-chip oscillator circuit arranged to generate and output the referenceclock CK_REF. The reference clock CK_REF may act as a system clock ofthe wireless system 200. Hence, the reference clock CK_REF generatedfrom the active oscillator 214 may be used to create periodical signalsneeded by normal operations of other on-chip components.

In this embodiment, pin(s) of the chip 201 are not coupled to anoff-chip oscillator when the wireless system 200 is in a normaloperation. For example, the off-chip oscillator is a crystal oscillatorwhich uses the mechanical resonance of a vibrating crystal ofpiezoelectric material to create an electrical signal with a precisefrequency. In other words, a normal operation of the wireless system 200can be achieved with the use of the internal reference clock CK_REFprovided by the on-chip oscillator (i.e., active oscillator 102 that isa crystal-less oscillator), and does not require an external referenceclock supplied from the off-chip oscillator such as a typical crystaloscillator. Since the off-chip oscillator can be omitted in anapplication device that uses the proposed wireless system 200, the BOMcost and the PCB area of the application device using the proposedwireless system 200 can be reduced.

In this embodiment, a reference frequency of the reference clock CK_REFgenerated from the active oscillator 214 may be different from (e.g.,higher than or lower than) an LO frequency needed by the TX circuit 204and the RX circuit 206. Hence, the frequency synthesizer 216 is afrequency processing circuit designed to process the reference clockCK_REF for creating the LO signal S_LO with the needed LO frequency. Forexample, the frequency synthesizer 216 may include a phase-locked loop(PLL) circuit, a frequency multiplier circuit, and/or a frequencydivider circuit, depending upon the discrepancy between the LO frequencyof the LO signal S_LO and the reference frequency of the reference clockCK_REF.

The TX circuit 204 and the RX circuit 206 may share the same off-chipantenna (e.g., antenna 105 shown in FIG. 1) through the switch circuit208 under the control of the control circuit 210. Specifically, theswitch circuit 208 is a transmit/receive (TR) switch that is capable ofalternately connecting the TX circuit 204 and the RX circuit 206 ashared antenna. When the wireless system 200 operates under a TX mode,the control circuit 210 may turn off the RX circuit 206, and may furtherinstruct the switch circuit 208 to couple an output port of the TXcircuit 204 to the off-chip antenna (e.g., antenna 105 shown in FIG. 1).When the wireless system 200 operates under an RX mode, the controlcircuit 210 may turn off the TX circuit 204, and may further instructthe switch circuit 208 to couple an input port of the RX circuit 206 tothe off-chip antenna (e.g., antenna 105 shown in FIG. 1).

The TX circuit 204 is used to perform an up-conversion process thatconverts a TX signal from a lower frequency to a higher frequency forsignal transmission via the off-chip antenna. The RX circuit 206 is usedto receive an RX signal from the off-chip antenna, and perform adown-conversion process that converts the RX signal from a higherfrequency to a lower frequency for signal reception. The LO frequency ofthe LO signal S_LO should be properly set to meet requirements of theup-conversion process and the down-conversion process. In a case wherethe wireless system 200 is a radar system (e.g., an automotive radarsystem or a non-automotive radar system), the LO signal S_LO may havethe LO frequency at 24 GHz, 60 GHz, 77 GHz, or 79 GHz. However, this isfor illustrative purposes only, and is not meant to be a limitation ofthe present invention.

Compared to an external reference clock generated from an off-chipcrystal oscillator, an internal reference clock generated from anon-chip active oscillator may have less stability and accuracy. Hence,the present invention further proposes a frequency calibration schemefor calibrating a frequency error of the LO signal S_LO that mainlyresults from unstability and inaccuracy of the on-chip active oscillator214. After the LO frequency of the LO signal S_LO is properlycalibrated, a frequency error of a TX signal transmitted over the aircan be reduced, and a transmit frequency of the TX signal can pass theFCC/ETSI emission regulation.

The frequency error may be calibrated in a final test (FT) stage of theassembly line, or may be calibrated in a mass production (MP) line. Whenthe proposed frequency calibration scheme is enabled, the wirelesssystem 200 is controlled to operate in the RX mode, where the switchcircuit 208 couples an input port of the RX circuit 206 to the off-chipantenna, and the TX circuit 202 is turned off to avoid LO signalleakage.

An in-phase/quadrature (I/Q) modulation scheme may be employed by thewireless system 200 for signal transmission, and an I/Q demodulationscheme may be employed by the wireless system 200 for signal reception.Taking the RX circuit 206 for example, it includes a quadrature phasesplitter 222, an in-phase mixer 224, and a quadrature mixer 226. The RXcircuit 206 generates a down-converted RX signal by performingdown-conversion upon an RX input signal RX_IN according to the LO signalS_LO. The down-converted RX signal includes a down-converted in-phasesignal RX_I and a down-converted quadrature signal RX_Q. The quadraturephase splitter 222 is arranged to generate an in-phase LO signal LO_Iand a quadrature LO signal LO_Q according to the LO signal S_LO. Forexample, the LO signal S_LO and the in-phase LO signal LO_I may have thesame frequency and the same phase; and the LO signal S_LO and thequadrature LO signal LO_Q may have the same frequency and a 90-degreephase difference. The in-phase mixer 224 is arranged to mix the RX inputsignal RX_IN and the in-phase LO signal LO_I to generate thedown-converted in-phase signal RX_I. The quadrature mixer 226 isarranged to mix the RX input signal RX_IN and the quadrature LO signalLO_Q to generate the down-converted quadrature signal RX_Q.

Since I/Q demodulation is used by the RX circuit 206, either of thepositive frequency tone and the negative frequency tone can be detected.Hence, the processing circuit 212 may act as a calibration circuit usedto process the down-converted RX signal (RX_I, RX_Q) for frequencycalibration. For example, the processing circuit 212 may be an on-chipmicrocontroller unit (MCU) or an on-chip radar signal processor (RSP).The processing circuit 212 is arranged to generate a frequencycalibration control output S_CTRL according to a signal characteristicof the down-converted RX signal (RX_I, RX_Q), and output the frequencycalibration control output S_CTRL to the LO signal generation circuit202. For example, the frequency calibration control output S_CTRL may bea calibration signal output or a calibration data output. The LO signalgeneration circuit 202 adjusts the LO frequency of the LO signal S_LO inresponse to the frequency calibration control output S_CTRL. In thisembodiment, the processing circuit 212 may check the signalcharacteristic of the down-converted RX signal (RX_I, RX_Q) by detectingexistence of a calibration tone within a receiver IF bandwidth centeredat the current LO frequency (which is a direct current (DC) frequency ofthe receiver IF bandwidth). Specifically, the calibration signal source102 shown in FIG. 1 transmits the calibration reference signal S_CALwith the calibration tone (e.g., an IF tone having a CW frequency f₀with approximate zero ppm frequency error). The down-converted RX signal(RX_I, RX_Q) provides a representation of the IF signal. When the LOfrequency of the LO signal S_LO is close to the CW frequency f₀, thecalibration tone (e.g., IF tone having the CW frequency f₀) can be foundin the output spectrum of the down-converted RX signal (RX_I, RX_Q). Inother words, existence of the calibration tone within the receiver IFbandwidth centered at the current LO frequency indicates that thecurrent LO frequency of the LO signal S_LO is close to the CW frequencyf₀. Hence, when the wireless system 200 operates in the TX mode, the TXcircuit 204 using the current LO frequency of the LO signal S_LO canachieve signal transmission without frequency error or with a smallfrequency error.

When the LO frequency of the LO signal S_LO is largely deviated from thetarget LO frequency, the calibration tone (e.g., IF tone having the CWfrequency f₀) is beyond the receiver IF bandwidth, and cannot be foundin the output spectrum of the down-converted RX signal (RX_I, RX_Q). Inother words, absence of the calibration tone within the receiver IFbandwidth centered at the current LO frequency indicates that thecurrent LO frequency of the LO signal S_LO is largely deviated from thetarget LO frequency. The processing circuit 212 generates the frequencycalibration control output S_CTRL for frequency calibration of the LOsignal S_LO.

In a first exemplary calibration design, the active oscillator 214receives the frequency calibration control output S_CTRL, and adjuststhe reference frequency of the reference clock CK_REF according to thefrequency calibration control output S_CTRL. It should be noted that thefrequency synthesizer 216 may not adjust the reference frequency of thereference clock CK_REF in response to the frequency calibration controloutput S_CTRL. Since the LO signal S_LO is derived from the referenceclock CK_REF, the LO frequency of the LO signal S_LO is calibrated dueto the frequency calibration of the reference clock CK_REF.

In a second exemplary calibration design, the frequency synthesizer 216receives the frequency calibration control output S_CTRL, and adjustsits synthesizer setting according to the frequency calibration controloutput S_CTRL. Hence, the LO frequency of the LO signal S_LO iscalibrated due to the adjusted synthesizer setting. It should be notedthat the active oscillator 214 may not adjust the reference frequency ofthe reference clock CK_REF in response to the frequency calibrationcontrol output S_CTRL.

In a third exemplary calibration design, the frequency calibrationcontrol output S_CTRL is supplied to both of the active oscillator 214and the frequency synthesizer 216. Hence, the active oscillator 214receives the frequency calibration control output S_CTRL, and adjuststhe reference frequency of the reference clock CK_REF according to thefrequency calibration control output S_CTRL. In addition, the frequencysynthesizer 216 receives the frequency calibration control outputS_CTRL, and adjusts its synthesizer setting according to the frequencycalibration control output S_CTRL. Since the LO signal S_LO is derivedfrom the reference clock CK_REF according to the synthesizer setting,the LO frequency of the LO signal S_LO is calibrated due to thefrequency calibration of the reference clock CK_REF and the adjustedsynthesizer setting.

Please refer to FIG. 3 in conjunction with FIG. 4. FIG. 3 is a diagramillustrating the relationship between the LO frequency of the LO signalS_LO and the control code OSC_code of the active oscillator 214according to an embodiment of the present invention. FIG. 4 is a diagramillustrating an output spectrum of the down-converted RX signalaccording to an embodiment of the present invention. The calibrationreference signal S_CAL with the calibration tone (e.g., IF tone havingthe CW frequency f₀ with approximate zero ppm frequency error) istransmitted from the calibration signal source 102 for frequencycalibration of the wireless system 104/200. For example, the CWfrequency f₀ is set by a value equal to a target LO frequency. Hence,one objective of the proposed frequency calibration is to make the LOfrequency of the LO signal S_LO close to f₀.

As shown in FIG. 3, the LO frequency of the LO signal S_LO can beadjusted by changing the control code OSC_code from 1 to N. Theadjustment frequency range FR should be assured to cover the desired CWfrequency f₀. The calibration tone at the CW frequency f₀ can bedown-converted to an IF signal (i.e., down-converted RX signalconsisting of the in-phase RX signal RX_I and the quadrature RX signalRX_Q), and the LO frequency is controlled by the control code OSC_codeof the active oscillator 214. When OSC_code=1, the calibration tone atthe CW frequency f₀ is beyond an output spectrum having the receiver IFbandwidth BW_IF and centered at the current LO frequency set by thecontrol code OSC_code. Since the calibration tone cannot be found in theoutput spectrum of the down-converted RX signal (RX_I, RX_Q), thefrequency calibration control output S_CTRL is adjusted by increasingthe control code OSC_code.

When OSC_code=2, the calibration tone at the CW frequency f₀ is beyondan output spectrum having the receiver IF bandwidth BW_IF and centeredat the current LO frequency set by the control code OSC_code. Since thecalibration tone is still not found in the output spectrum of thedown-converted RX signal (RX_I, RX_Q), the frequency calibration controloutput S_CTRL is further adjusted by increasing the control codeOSC_code.

When OSC_code=3, the calibration tone at the CW frequency f₀ is beyondan output spectrum having the receiver IF bandwidth BW_IF and centeredat the current LO frequency set by the control code OSC_code. Since thecalibration tone is still not found in the output spectrum of thedown-converted RX signal (RX_I, RX_Q), the frequency calibration controloutput S_CTRL is further adjusted by increasing the control codeOSC_code.

When OSC_code=4, the calibration tone at the CW frequency f₀ is withinan output spectrum having the receiver IF bandwidth BW_IF and centeredat the current LO frequency set by the control code OSC_code, asillustrated in FIG. 4. Since the calibration tone is successfullydetected in the output spectrum of the down-converted RX signal (RX_I,RX_Q), the optimized control code (e.g., OSC_code=4) is found, and thefrequency calibration procedure is done. When the optimized control code(e.g., OSC_code=4) is used under a normal operation of the wirelesssystem 104/200, the LO frequency of the LO signal S_LO is ensured to beclose to the CW frequency f₀. Hence, the TX signal transmitted from theoff-chip antenna (e.g., antenna 105 shown in FIG. 1) is ensured to havean accurate RF mW (microwave) frequency or an accurate mmWave(millimeter wave) frequency.

It should be noted that the observation frequency region is dependent onthe receiver IF bandwidth BW_IF. Hence, the receiver IF bandwidth BW_IFshould be assured to be larger than the oscillator tuning resolution RS.

The calibration signal source 102 may be a standard signal source usedfor transmitting the calibration reference signal S_CAL with the CWfrequency f₀ for frequency calibration of the wireless system 104. In analternative design, the calibration signal source 102 may be implementedusing one of DUTs to serve as a golden sample. FIG. 5 is a diagramillustrating a second calibration system according to an embodiment ofthe present invention. The major difference between the calibrationsystems 100 and 500 is that the calibration system 500 uses a wirelesssystem 502 as a calibration signal source, where the wireless systems104 and 502 have the same circuit architecture. For example, thewireless systems 104 and 502 are identical radar system chips. Like thecalibration signal source 102 shown in FIG. 1, the wireless system 502is used to transmit a calibration reference signal S_CAL via an antenna503. For example, the calibration reference signal S_CAL is a continuouswaveform (CW) signal with a calibration tone (e.g., a CW frequency f₀with approximate zero ppm frequency error). To achieve the CW frequencyf₀ with approximate zero ppm frequency error, an external high-precisionoscillator 504 is connected to the wireless system 502 during thecalibration procedure. Since an accurate external reference clock issupplied from the external high-precision oscillator 504 to the wirelesssystem 502, the wireless system 502 can generate the calibrationreference signal S_CAL according to the accurate external referenceclock. For example, the external high-precision oscillator 504 may be anelectromechanical resonator based oscillator such as a temperaturecompensated crystal oscillator (TCXO).

FIG. 6 is a diagram illustrating a second wireless system according toan embodiment of the present invention. Each of the wireless systems 104and 502 shown in FIG. 5 may be implemented using the wireless system 600shown in FIG. 6. For example, the wireless system 600 may be a RadioDetection and Ranging (radar) system, such as a frequency modulatedcontinuous waveform (FMCW) radar system, a phase modulated continuouswave (PMCW) radar system, a micro-Doppler radar system, or apulse-Doppler radar system. For another example, the wireless system 600may be an automotive system such as an automotive radar system. In thisembodiment, the wireless system 600 is implemented on a chip 601, andtherefore has a plurality of on-chip components. The major differencebetween the wireless systems 200 and 600 is that the wireless system 600further supports the use of an off-chip oscillator during a frequencycalibration procedure. As shown in FIG. 6, the LO signal generationcircuit 602 includes a switch circuit (denoted by “SW”) 604. The switchcircuit 604 is coupled to the active oscillator 214 for receiving theinternal reference clock CK_REF. When the wireless system 600 is notselected as a golden sample (e.g., wireless system 104 shown in FIG. 5),no off-chip oscillator is connected to pin(s) of the chip 601, and theswitch circuit 604 is controlled by the processing circuit (e.g., RSP orMCU) 212 to output the internal reference clock CK_REF to the frequencysynthesizer 216. The wireless system 600 is controlled to operate in aTX mode during the frequency calibration procedure, and theaforementioned on-chip frequency calibration is performed to calibratethe LO frequency of the LO signal S_LO.

When the wireless system 600 is selected as a golden sample (e.g.,wireless system 502 shown in FIG. 5), an off-chip oscillator (e.g.,external high-precision oscillator 504 shown in FIG. 5) is connected topin(s) of the chip 601, and the switch circuit 604 is further coupled tothe off-chip oscillator for receiving an external reference clockCK_EXT. Since the wireless system 600 is used as a golden sample fortransmitting the calibration reference signal S_CAL, the wireless system600 is controlled to operate in a TX mode during the frequencycalibration procedure. Hence, the control circuit 210 may turn off theRX circuit 206, and may further instruct the switch circuit 208 tocouple an output port of the TX circuit 204 to the off-chip antenna(e.g., antenna 503 shown in FIG. 5). In addition, the switch circuit 604is controlled by the processing circuit (e.g., RSP or MCU) 212 to outputthe external reference clock CK_EXT to the frequency synthesizer 216.For example, the LO signal S_LO generated from the frequency synthesizer216 may be a CW signal with a CW frequency f₀. In this way, thecalibration reference signal S_CAL with the calibration tone (e.g., CWfrequency f₀ with approximate zero ppm frequency error) can betransmitted via the off-chip antenna (e.g., antenna 503 shown in FIG.5).

When there is more than one DUT (e.g., more than one wireless system104), a calibration reference signal transmitted from a calibrationsignal source (e.g., calibration signal source 102 shown in FIG. 1 orwireless system 502 shown in FIG. 5) can be received by multiple DUTsfor frequency calibration. In other words, a calibration system can usea single calibration signal source for achieving simultaneous frequencycalibration of multiple DUTs.

FIG. 7 is a diagram illustrating a calibration setup environmentaccording to an embodiment of the present invention. The calibrationsetup environment may be used for production line calibration. Forexample, each of the wireless systems S1, S2, S3, and S4 may beimplemented using the wireless system 600 shown in FIG. 6. The wirelesssystem S1 is a DUT selected as a golden sample, such that an externalreference clock CK_EXT provided from an off-chip oscillator is used forgenerating and transmitting a calibration reference signal S_CAL tomultiple wireless systems S2, S3, and S4 under test. During thefrequency calibration procedure, the wireless system S1 is controlled tooperate in a TX mode, while each of the wireless systems S2, S3, and S4is controlled to operate in an RX mode. The calibration reference signalS_CAL may be transmitted from the golden sample to one DUT in aline-of-sight manner or a multi-path reflection manner. Since thewireless systems S2, S3, and S4 have the same circuit architecture, thesame frequency calibration procedure mentioned above can be performed byeach of the wireless systems S2, S3, and S4 according to the calibrationreference signal S_CAL with the calibration tone. In this way, LOfrequencies of LO signals generated from LO signal generation circuitsof the wireless systems S2, S3, and S4 can be calibrated simultaneously.

Alternatively, the calibration setup environment may be used for asatellite sensor application. The wireless systems S1, S2, S3, and S4may be implemented indifferent radar sensors that are installed in aspace for object detection with good coverage. All the radar sensors canoperate in a Frequency-division multiple access (FDMA) mode or aTime-staggered frequency modulated continuous waveform (TS-FMCW) mode.They need frequency calibration to avoid frequency overlap interferencedue to each sensor's frequency error. For example, each of the wirelesssystems S1, S2, S3, and S4 may be implemented using the wireless system200 shown in FIG. 2. The wireless system S1 is selected as a masterdevice for generating and transmitting a calibration reference signalS_CAL to multiple wireless systems S2, S3, and S4. For example, thecalibration reference signal S_CAL may be transmitted from the masterdevice to one DUT in a line-of-sight manner or a multi-path reflectionmanner. Since the wireless system S1 is used as a master device fortransmitting the calibration reference signal S_CAL, the wireless systemS1 with the circuit architecture shown in FIG. 2 is controlled tooperate in a TX mode. Hence, the control circuit 210 may turn off the RXcircuit 206, and may further instruct the switch circuit 208 to couplean output port of the TX circuit 204 to the off-chip antenna (e.g.,antenna 503 shown in FIG. 5). In addition, the internal reference clockCK_REF is used by the frequency synthesizer 216 to generate the LOsignal S_LO, where the LO signal S_LO may be a CW signal with a CWfrequency f₀. In this way, the calibration reference signal S_CAL with acalibration tone (e.g., a CW frequency f₀+Δf, where Δf is a frequencyoffset resulting from the internal reference clock) can be transmittedvia the off-chip antenna (e.g., antenna 103 shown in FIG. 1). During thefrequency calibration procedure, the wireless system S1 is controlled tooperate in a TX mode, while each of the wireless systems S2, S3, and S4is controlled to operate in an RX mode. Since the wireless systems S2,S3, and S4 have the same circuit architecture, the same frequencycalibration procedure mentioned above can be performed by each of thewireless systems S2, S3, and S4 according to the calibration referencesignal S_CAL with the calibration tone. In this way, the wirelesssystems S2 finds a specific oscillator control code that corresponds tothe calibration tone under a calibration mode, and refers to thespecific oscillator control code found in the calibration mode to selectand use another oscillator control code under a normal operation mode,thereby avoiding the undesired frequency overlap interference ofwireless systems S1-S4. Similarly, the wireless systems S3 finds aspecific oscillator control code that corresponds to the calibrationtone under a calibration mode, and refers to the specific oscillatorcontrol code found in the calibration mode to select and use anotheroscillator control code under a normal operation mode, thereby avoidingthe undesired frequency overlap interference of wireless systems S1-S4;and the wireless systems S4 finds a specific oscillator control codethat corresponds to the calibration tone under a calibration mode, andrefers to the specific oscillator control code found in the calibrationmode to select and use another oscillator control code under a normaloperation mode, thereby avoiding the undesired frequency overlapinterference of wireless systems S1-S4.

As shown in FIG. 2/FIG. 6, the frequency calibration function can beintegrated into the wireless system 200/600 in the chip 201/601. Thus,no expensive RF mW (microwave) test instrument or mmWave (millimeterwave) test instrument is needed, and no extra personal computer (PC) ormicrocontroller unit (MCU) is needed to read data from the testinstrument and calibrate a frequency error by controlling the internaloscillator and/or internal frequency synthesizer. Further, since thesame frequency calibration function can be integrated into each of aplurality of wireless systems 200/600, multiple wireless systems 200/600can be calibrated at the same time. Moreover, one of the wirelesssystems 600 can be configured to receive an external reference clockfrom an external oscillator for acting as a golden sample whichgenerates and transmits a calibration reference signal to the rest ofthe wireless systems 600, a standard signal source for generating andtransmitting a calibration reference signal can be omitted. The proposedfrequency calibration scheme can be easily implemented in a final test(FT) of the assembly line, or can be easily implemented in a massproduction (MP) line.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A wireless system comprising: a local oscillator(LO) signal generation circuit, arranged to generate an LO signalaccording to a reference clock, wherein the LO signal generation circuitcomprises: an active oscillator, arranged to generate the referenceclock, wherein the active oscillator comprises at least one activecomponent, and does not include an electromechanical resonator; areceiver (RX) circuit, arranged to generate a down-converted RX signalby performing down-conversion upon an RX input signal according to theLO signal; and a calibration circuit, arranged to generate a frequencycalibration control output according to a signal characteristic of thedown-converted RX signal, and output the frequency calibration controloutput to the LO signal generation circuit, wherein the LO signalgeneration circuit adjusts an LO frequency of the LO signal in responseto the frequency calibration control output.
 2. The wireless system ofclaim 1, wherein the wireless system is implemented on a chip, and theactive oscillator is an on-chip oscillator.
 3. The wireless system ofclaim 1, wherein the active oscillator is arranged to receive thefrequency calibration control output, and adjust a reference frequencyof the reference clock according to the frequency calibration controloutput.
 4. The wireless system of claim 1, wherein the down-converted RXsignal comprises a down-converted in-phase signal and a down-convertedquadrature signal, and the RX circuit comprises: a quadrature phasesplitter, arranged to generate an in-phase LO signal and a quadrature LOsignal according to the LO signal; an in-phase mixer, arranged to mixthe RX input signal and the in-phase LO signal to generate thedown-converted in-phase signal; and a quadrature mixer, arranged to mixthe RX input signal and the quadrature LO signal to generate thedown-converted quadrature signal.
 5. The wireless system of claim 4,wherein the calibration circuit is arranged to check if a calibrationtone exists in the down-converted RX signal, and is further arranged toset the frequency calibration control output for changing the LOfrequency of the LO signal in response to a condition that thecalibration tone is not found in the down-converted RX signal.
 6. Thewireless system of claim 5, wherein under the condition that thecalibration tone is not found in the down-converted RX signal, theactive oscillator receives the frequency calibration control output, andadjusts a reference frequency of the reference clock according to thefrequency calibration control output.
 7. The wireless system of claim 1,wherein the wireless system is a Radio Detection and Ranging (radar)system.
 8. The wireless system of claim 1, wherein the wireless systemis an automotive system.
 9. A calibration system comprising: acalibration signal source, arranged to transmit a calibration referencesignal via an antenna; and a first wireless system comprising: a firstlocal oscillator (LO) signal generation circuit, arranged to generate afirst LO signal according to a first reference clock, wherein the firstLO signal generation circuit comprises: a first active oscillator,arranged to generate the first reference clock, wherein the first activeoscillator comprises at least one active component, and does not includean electromechanical resonator; a first receiver (RX) circuit, arrangedto generate a first down-converted RX signal by performingdown-conversion upon a first RX input signal according to the first LOsignal, wherein the first RX input signal is obtained from thecalibration reference signal received via an antenna; and a firstcalibration circuit, arranged to generate a first frequency calibrationcontrol output according to a signal characteristic of the firstdown-converted RX signal, and output the first frequency calibrationcontrol output to the first LO signal generation circuit, wherein thefirst LO signal generation circuit adjusts an LO frequency of the firstLO signal in response to the first frequency calibration control output.10. The calibration system of claim 9, wherein the first activeoscillator is arranged to receive the first frequency calibrationcontrol output, and adjust a reference frequency of the first referenceclock according to the first frequency calibration control output. 11.The calibration system of claim 9, wherein the calibration referencesignal is a continuous waveform (CW) signal.
 12. The calibration systemof claim 9, wherein the calibration signal source comprises: a secondlocal oscillator (LO) signal generation circuit, arranged to generate asecond LO signal according to a second reference clock, wherein thesecond reference clock is generated from a reference oscillator thatcomprises an electromechanical resonator; and a transmitter (TX)circuit, arranged to transmit the calibration reference signal accordingto the second LO signal.
 13. The calibration system of claim 12, whereinthe second LO signal generation circuit further comprises: a secondactive oscillator, wherein the second active oscillator comprises atleast one active component, and does not include an electromechanicalresonator; and a switch circuit, coupled to the reference oscillator andthe second active oscillator, wherein the switch circuit is arranged toselect an output of the reference oscillator as the second referenceclock.
 14. The calibration system of claim 9, wherein the calibrationsignal source comprises: a second local oscillator (LO) signalgeneration circuit, arranged to generate a second LO signal according toa second reference clock, wherein the second LO signal generationcircuit comprises: a second active oscillator, arranged to generate andoutput the second reference clock, wherein the second active oscillatorcomprises at least one active component, and does not include anelectromechanical resonator; and a transmitter (TX) circuit, arranged totransmit the calibration reference signal according to the second LOsignal.
 15. The calibration system of claim 9, further comprising: asecond wireless system comprising: a second local oscillator (LO) signalgeneration circuit, arranged to generate a second LO signal according toa second reference clock, wherein the second LO signal generationcircuit comprises: a second active oscillator, arranged to generate thesecond reference clock, wherein the second active oscillator comprisesat least one active component, and does not include an electromechanicalresonator; a second receiver (RX) circuit, arranged to generate a seconddown-converted RX signal by performing down-conversion upon a second RXinput signal according to the second LO signal, wherein the second RXinput signal is obtained from the calibration reference signal receivedvia an antenna; and a second calibration circuit, arranged to generate asecond frequency calibration control output according to a signalcharacteristic of the second down-converted RX signal, and output thesecond frequency calibration control output to the second LO signalgeneration circuit, wherein the second LO signal generation circuitadjusts an LO frequency of the second LO signal in response to thesecond frequency calibration control output.
 16. The calibration systemof claim 9, wherein the first wireless system is a Radio Detection andRanging (radar) system.
 17. The calibration system of claim 9, whereinthe first wireless system is an automotive system.
 18. A localoscillator (LO) signal calibration method comprising: generating an LOsignal according to a reference clock, wherein the reference clock isgenerated by an active oscillator, and the active oscillator comprisesat least one active component, and does not include an electromechanicalresonator; generating a down-converted RX signal by performingdown-conversion upon an RX input signal according to the LO signal; andgenerating a frequency calibration control output according to a signalcharacteristic of the down-converted RX signal, wherein an LO frequencyof the LO signal is adjusted in response to the frequency calibrationcontrol output.
 19. The LO signal calibration method of claim 18,further comprising: outputting the frequency calibration control outputto the active oscillator, wherein the active oscillator adjusts areference frequency of the reference clock in response to the frequencycalibration control output.
 20. The LO signal calibration method ofclaim 18, wherein the wireless system is a Radio Detection and Ranging(radar) system.
 21. The LO signal calibration method of claim 18,wherein the wireless system is an automotive system.